Electron Holography of Barrier Structures in Co/ZrAlO_x/Co Magnetic Tunnel Junctions

We investigate the potential profiles and elemental distribution of barriers in Co/ZrAlO_x/Co magnetic tunnel junctions (MTJs) using electron holography (EH) and scanning transmission electron microscopy. The MTJ barriers are introduced by oxidizing a bilayer consisting with a uniform 0.45-nm Al layer and a wedge-shaped Zr layer (0-2nm). From the scanning transmission electron microscopy, AlO_x and ZrO_x layers are mixed together, indicating that compact AlO_x layer cannot be formed in such a bilayer structure of barriers. The EH results reveal that there are no sharp interfaces between the barrier and magnetic electrodes, which may be responsible for a smaller tunnelling magnetoresistance compared with the MTJs of Co/AlO_x/Co.